Squid giant axons will be microinjected with both aequorin and with phenol red, a dye whose isobestic absorption is very close to the emission peak for light from aequorin. The concentration of the dye ultimately resulting in the axon will be of the order of 10 mM and at this concentration a substantial fraction of the photons emitted from the core of the axoplasm will be absorbed before reaching the outside. This experimental treatment biases the gbservational region for aequorin to approximately the first 25 um of axoplasm just underneath the excitable membrane. Such aequorin and phenol red injected axons will be examined under voltage clamp conditions for chages in Ca entry that are related to bioelectric phenomena. In particular, attention will be given to the relative contribution that depolarization makes to Ca entry when the axon is depleted of Nai (by stimulation in Li seawater) as compared with axons with a normal Nai content. Measurements should enable one to judge whether there is in fact an appreciable entry of Ca "late" channels compared with Ca entry through Na channels and similar entry via the Na/Ca exchange system.